Every half a millennium or so, waves of change rock Christianity until they cause the kind of earthquake that forces historians to start using capital letters.

"What happened before the Great Reformation, we all know," said Phyllis Tickle, author of "God Talk in America" and two dozen books on faith and culture. "We know, for instance, that some sucker sailed west and west and west and didn't fall off the dad gum thing. That was a serious blow."

So Columbus sailed the ocean blue in 1492 and then a flat, neatly stacked universe flipped upside down. Soon, people were talking about nation states, the decline of landed gentry, the rise of a middle class and the invention of a printing press with movable type. Toss in a monk named Martin Luther and you're talking Reformation -- with a big "R" -- followed by a Counter-Reformation.

Back up 500 years to 1054 and you have the Great Schism that separated Rome and from Eastern Orthodoxy. Back up another 500 years or so and you find the Fall of the Roman Empire. The transformative events of the first century A.D. speak for themselves.

Church leaders who can do the math should be looking over their shoulders about now, argued Tickle, speaking to clergy, educators and lay leaders at the recent National Youth Workers Convention in Atlanta.

After all, seismic changes have been rolling through Western culture for a century or more -- from Charles Darwin to the World Wide Web and all points in between. The result is a whirlwind of spiritual trends and blends, with churches splintering into a dizzying variety of networks and affinity groups to create what scholars call the post-denominational age.

Tickle is ready to call this the "Great Emergence," with a tip of her hat to the edgy flocks in the postmodern "emerging church movement."

"Emerging or emergent Christianity is the new form of Christianity that will serve the whole of the Great Emergence in the same way that Protestantism served the Great Reformation," she said, in a speech that mixed doses of academic content with the wit of a proud Episcopalian from the deeply Southern culture of Western Tennessee.

However, anyone who studies history knows that the birth of something new doesn't mean the death of older forms of faith. The Vatican didn't disappear after the Protestant Reformation.

This kind of revolution, said Tickle, doesn't mean "any one of those forms of earlier Christianity ever ceases to be. It simply means that every time we have one of these great upheavals ... whatever was the dominant form of Christianity loses its pride of place and gives way to something new. What's giving way, right now, is Protestantism as you and I have always known it."

It helps to think of dividing American Christianity, she said, into four basic streams -- liturgical, Evangelical, Pentecostal-charismatic and old, mainline Protestant. The problem, of course, is that there are now charismatic Episcopalians and Catholics, as well as plenty of Evangelicals who are interested in liturgical worship and social justice. Conservative megachurches are being forced to compromise because of sobering changes in marriage and family life, while many progressive flocks are being blasted apart by conflicts over the same issues.

In other words, the lines are blurring between once distinct approaches to faith. Tickle is convinced that 60 percent of American Christians are worshipping in pews that have, to one degree or another, been touched by what is happening in all four camps. At the same time, each of the quadrants includes churches -- perhaps 40 percent of this picture -- that are determined to defend their unique traditions no matter what.

The truly "emerging churches" are the ones that are opening their doors at the heart of this changing matrix, she said. Their leaders are determined not to be sucked into what they call "inherited church" life and the institutional ties that bind. They are willing to shed dogma and rethink doctrine, in an attempt to tell the Christian story in a new way.

"These emergent folks are enthusiastically steering toward the middle and embracing the whole post-denominational world," said Tickle. "We could end up with something like a new form of Pan-Protestantism. ... It's all kind of exciting and scary at the same time, but we can take some comfort in knowing that Christianity has been through this before."